Tumors and other targets in the head, spine, abdomen, and lungs have been successfully treated using radiosurgery. During radiosurgery, a series of beams of ionizing radiation are often directed from outside a patient so as to converge at a target region, with the radiation beams often comprising MeV X-ray beams fired from different positions and orientations. The beams can be directed through intermediate tissue toward the target tissue so as to alter the biology of a tumor. The beam trajectories help limit the radiation exposure to the intermediate and other collateral tissues, while the cumulative radiation dose at the target can treat the tumor. The CYBERKNIFE radiosurgical system (Accuray Inc.) and the TRILOGY radiosurgical system (Varian Medical Systems) are two known radiosurgical treatment systems.
Modern radiosurgical systems incorporate imaging into the treatment system so as to verify the position of the target tissue and adjust to minor patient movements. Some systems also have an ability to treat tissues that move during respiration, and this feature has significantly broadened the number of patients that can benefit from radiosurgery.
Radiosurgical treatments of other tissues that undergo physiological movements have also been proposed, including the directing of radiation toward selected areas of the heart for treatment of atrial fibrillation and other arrhythmias. During atrial fibrillation, the atria lose their organized pumping action. In a healthy sinus rhythm, the atria contract, the valves open, and blood fills the ventricles or lower chambers. The ventricles then contract to complete an organized cycle of each heart beat. Atrial fibrillation, in contrast, has been characterized as a storm of electrical energy that travels across the atria causing the upper chambers of the heart to quiver or fibrillate. During atrial fibrillation, the blood is not able to empty sufficiently from the atria into the ventricles with each heartbeat. By directing ionizing radiation toward the heart based on appropriate lesion patterns, the resulting scar tissue may prevent recirculating electrical signals and thereby diminish or eliminate the atrial fibrillation.
In standard radiosurgical treatments of tumors and the like, computed tomography (CT) imaging provides a series of planar X-ray scans. For the X-rays adjacent a tumor, the planning physician draws a boundary of the target tissue, with the boundary being drawn on the scan traversing through the tumor and the boundary encompassing the tumor (and typically including some additional offset or margin of treated tissue for safety). As the tumor is typically contained within one organ (but may alternatively extend beyond the organ surface to an adjacent organ) the planned treatment boundary is fairly independent of tissue/tissue interface contours. Hence, the treatment plan is typically drawn up as a series of circles surrounding the tumor on each CT scan in which the tumor is visible.
It is difficult to draw an appropriate arrhythmia lesion treatment plan for forming patterns on conventional planar CT scans using standard radiosurgical planning interfaces. A physician must evaluate the multiple CT scans, and draw appropriate lines and/or circles representing a treatment plan at each planar slice of the heart. The physician must be able to visualize desired treatment areas from each planer scan. While this appears to be a mere inconvenience, work in connection with the present invention indicates it is surprisingly difficult to efficiently establish an arrhythmia treatment plan using existing radiosurgical treatment planning tools in light of the geometry of the heart.